PHASE-FIELD SIMULATION ON THE FUNCTIONAL PROPERTIES OF STRESS-ASSISTED AGING NiTi SHAPE MEMORY ALLOYS

In this work, an improved phase field model coupling precipitation and martensitic transformation (MT) was used to simulate the superelasticity, elastocaloric effect, one-way shape memory effect (OWSME) and stress-assisted two-way shape memory effect (SATWSME) of the stress-assisted aging NiTi single crystal. The influence of single-orientated Ni₄Ti₃ precipitates on the functional properties and the underlying microscopic mechanism were revealed. The simulation results show that due to the strong geometric constraints of single-oriented Ni₄Ti₃ precipitates, the MT in the superelastic or SATWSME process is not realized by the rapid widening of martensitic bands, but by the nucleation and growth of a large number of needlelike martensite phases, and the reverse MT is achieved by the reduction and disappearance of a large number of needlelike martensite phases. The single-crystal system containing single-oriented Ni₄Ti₃ precipitates shows an excellent elastocaloric effect. For the OWSME, in the single-crystal system with single-oriented Ni₄Ti₃ precipitates, the twinned interfaces formed during the temperature-induced MT are parallel to the inertial plane of the Ni₄Ti₃ precipitates; the constraints of the single-orientated and different-orientated Ni₄Ti₃ precipitates on the martensitic reorientation are different, and those on the temperature-induced reverse MT are also different, resulting in the difference in the stress-strain-temperature curves of the two cases. This work will provide insights for understanding the effect of Ni₄Ti₃ precipitates on the functional properties of NiTi alloys, and provide references for regulating the MT, functional and mechanical properties of such alloys.